Relays have been used to expand the coverage of conventional cellular systems. With such relays, a mobile station that is out of range of the base station may still be able to communicate with a base station via one of the relays. The relay has very little functionality, typically only re-transmitting signals received from the mobile station or from the base station.
MIMO (multiple input multiple output) systems feature multiple antennas at the transmitter and/or receiver, and spatial processing at the receiver to recover transmitted data. Examples of existing MIMO technologies include STBC (space-time block coding) and spatial multiplexing (SM) approaches.
With STBC (space-time block coding), each antenna transmits a respective stream, and there is some correlation between the streams, either due to coding and modulation of input data prior to block coding, or in the block coding structure per se. STBC schemes involve a little more complexity at the transmitter, but may allow simplified receiver complexity. An example of an STBC scheme that relies on coding and modulation prior to block coding is the so-called “BLAST” approach in which each transmit antenna is used to transmit a unique symbol stream, with coding and modulation being employed prior to block coding to introduce correlation. An example of an STBC scheme that relies on the block coding structure per se is STTD (space time transmit diversity) where each symbol appears on multiple antennas. A well-known STTD scheme is Alamouti code-based transmission.
With spatial multiplexing (SM), each antenna is used to transmit an independent data stream. There is no correlation introduced by coding and modulation. SM approaches have reduced transmitter complexity, but involve higher receiver complexity. Well known SM schemes include the so-called V-BLAST (vertical BLAST) and D-BLAST (diagonal BLAST) where independent symbol streams are transmitted on each antenna. With SM, independent data streams are transmitted over different antennas, to generate a multiplexing gain. When used with Maximum Likelihood decoding, such a scheme is found to provide good performance.
While traditional STBC exploits both the multiplexing gain as well as diversity gain, spatial multiplexing systems such as V-BLAST provide primarily a multiplexing gain. While the diversity gains levels off with increasing number of antennas, the spatial multiplexing gain increases linearly with the increase in number of antennas.
The benefit of MIMO is significant when the SINRs of the MIMO signals are comparable thereby allowing a full-rank MIMO channel realization. This restricts the number of instances where cooperative MIMO can be successfully employed in systems featuring distributed users having varying SINR conditions.
In systems employing cooperative MIMO, multiple mobile stations cooperatively transmit the data of a single mobile station so as to appear as a MIMO transmission. For example, two mobile stations with one antenna each can transmit one of the mobile stations data. A two antenna base station could then receive the two signals and process them using MIMO techniques. This scheme has some disadvantages. For example, it requires each mobile station's data to be exchanged between the two mobile stations to enable cooperative transmission. Furthermore, the transmission is opportunistic since it is based on access bandwidth in the peer mobile station over and above its own prioritized transmissions. The scheme adds complexity to the mobile station in that it requires an additional transceiver chain to transmit and receive data from its peers. Cooperative MIMO has been shown to provide significant capacity improvements in cellular systems. Since the exchange between two mobile stations is an essential component of cooperative MIMO, the mobile stations need to be conveniently located to exchange the information. Thus, the application of cooperative MIMO is limited to such scenarios.
Infrastructure based 2-hop relaying with the use of cellular spectrum for the relaying function has also been shown to provide significant coverage improvement in cellular systems, resulting in greater ubiquity of data rates as the user moves around the cell. Despite the fact that the bandwidth resource at the base station is now used for both the mobile station-to-relay transmissions and relay-to-base station transmissions, the improved SINR conditions on each of the two hops result in a higher aggregate SINR on the link as a whole and therefore improves the coverage to mobile stations that are further away from the base station.
FIG. 1 shows an example of conventional fixed infrastructure based selective relaying. Shown is a base station 10 having nominal coverage area 12. Fixed infrastructure relays 14,16 are also provided each with respective coverage areas 18,20. It can be seen that the relays serve to increase the coverage area of the base station. Mobile stations such as mobile station 22 that are within the coverage area 12 of the base station 10 can communicate directly with the mobile stations such as mobile stations 24 and 26 that are outside the coverage area of the base station 10, but that are within the coverage area of one of the relays such as relay 14, can communicate by first communicating to the relay 14 and then having their signals relayed from the relay 14 to the base station 10 as illustrated. The result is a multi-hop extension of cellular communication. Various FDD (frequency division duplexing)/TDM (time division multiplexing) approaches have been proposed for dealing with the transmission between the mobile stations and between relays and the base station. In a particular example illustrated at 30, a cellular base station 10, a relay 14 and a mobile station 24 communicate using combined FDD/TDD such that during a first time interval T1 the base station 10 and the relay 14 communicate using uplink and downlink frequencies fUL and fDL respectively while during a second time period T2 the mobile station 24 and the relay 14 communicate using uplink and downlink freqeuncies fDL and fUL.